Conventionally, the Huffman coding system is known as a data coding system of a file composed of data such as character data, etc., to be adopted in an information processing apparatus for processing a digital signal, such as a personal computer, etc.
In the Huffman coding system, a compression table of a coding system in which compressed codes are given to character data from the shortest bit string to the longest bit string in order from the highest appearance frequency (probability) is adopted. Referring to the compression table, the character data in the file is converted into the compressed codes to form a compressed file. Then, the compressed codes of the compressed file are restored to the original character data referring to the compression table.
Specifically, in the Huffman coding system, by expressing each data such as characters, etc., in the file of the document, etc., in such a manner that the character data which appears frequently is expressed using compressed codes of, for example, four bits, and the character data which appears less frequently is expressed using compressed codes of, for example, around 10 bits, the size of the compressed file can be significantly reduced.
However, in the described Huffman coding system, as the compression table is fixed, in the case of compressing the data in the file, composed of character data including many special characters, the data compression efficiency greatly drops.
To eliminate the described problem, for example, Japanese Laid-Open Patent Application No. 232626/1988 (Tokukaisho 63-232626) discloses a hardware based data compression/restoring system wherein an adaptive coding system in which a compression table varies according to the appearance frequency of each character data is adopted in replacement of the conventional static Huffman coding system using the fixed compression table.
According to the described conventional Huffman coding system, as the compression is carried out based on a unit of 8 bits, which is advantageous in processing data in the microprocessor, in the case of converting a minimum unit of 8-bit into the compressed code, the compression table would contain 256 kinds of compressed codes.
As described, in the conventional coding system, in the case of compressing character font data by the described Huffman coding system, if the compression is performed in the unit of 8-bit, the kinds of the compressed codes would be expanded to 256 kinds. However, the described coding system has such a drawback that the appearance frequency of each code would be broadly distributed (scattered), thereby presenting the problem that there is a limit in improving an efficiency in compressing the character font data.
When the described coding system of Japanese Laid-Open Patent Application No. 232626/1988 (Tokukaisho 63-232626) is applied to the character font data, the compression rate can be improved only within the known range, and an optimization of the data compression cannot be attained, thereby presenting the problem that there is a limit in improving the compression rate of the character font data.